213013-05-9Relevant academic research and scientific papers
Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(ii) complexes
Delgado-Rebollo, Manuela,Canseco-Gonzalez, Daniel,Hollering, Manuela,Mueller-Bunz, Helge,Albrecht, Martin
supporting information, p. 4462 - 4473 (2014/03/21)
We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h-1 for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.
N-Mesityl-C-acylketenimines: 1,5-Sigmatropic Shifts and Electrocyclization to Quinolines
Rao, V. V. Ramana,Fulloon, Belinda E.,Bernhardt, Paul V.,Koch, Rainer,Wentrup, Curt
, p. 5779 - 5786 (2007/10/03)
Flash vacuum thermolysis (FVT) of triazoles 6a- c generates α-oxoketenimines 10, the ester 10a being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a. Ketenes 9 and ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups under mild FVT conditions (ca. 350-400°C). Both 9 and 10 are directly observable by IR spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca. 400°C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal mol-1 (161 kJ mol-1) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal mol-1.
